Traditionally, pipelines were welded together offshore and these were laid on the sea bed using the S lay method down a stinger from a lay barge. The lay barge then crabbed along the lay route using an eight legged anchor spread at a speed of approximately 1 mile or 1.6 km per day. Lay barges were superseded by large dynamically positioned lay ships, which embarked double jointed pipes. Up to seven joints of pipe were welded together concurrently on multiple weld stations on the lay-ship. The result of this operation was that the pipeline was installed much quicker, the speed of advance now being 5 to 8 kilometers per day. As water depths increased, the S lay method was replaced by the vertical J lay method.
Faster pipeline installation can be achieved using a reeled pipeline method. In this application the 12.2 meter joints of pipe are welded together onshore in “spool-bases” and configured into continuous lengths of pipe known as ‘stalks’ which are approximately 1 km long. Stalk sections of pre-welded pipe are then wound onto the lay vessel's reel, in a reeling and stalk welding process, to form a continuous 20 km length on the pipe-lay vessel. The reeled pipe-lay vessel then sails to the field, and during a suitable weather window, reels the 20 km pipeline section onto the seabed. The empty reeled pipe-lay vessel then sails back to the spool-base, embarks another 20 km section of pipe, sails back to field, picks up the end of the first 20 km section of pipe, connects it onto the second section of pipe, and reels the second section of pipe onto the seabed, thereby extending the pipe-lay to 40 km. The reeled pipe-lay vessel repeats this process, until the desired length of pipeline is achieved. The speed of advance using this method is typically 15-18 km per day.
‘Safe’ reeled pipe-lay is a complex operation, and many factors must be considered and planned for throughout the pipe-lay process. Issues regarding the pipeline material include material grade, tensile strength, pipe diameter, wall thickness, ability to weld the material, pipe weight, and the coatings used to cover the pipe line material to prevent corrosion are extremely important. Further factors include pipeline buoyancy; the resistance of the pipeline to corrosion and/or internal erosion, resistance to hydrogen sulphide, resistance to high internal pressures and/or high temperatures; the hydrostatic pressure the pipeline will be exposed to during the lay; the circumferential roundness of the pipeline and its deviation from same; the bending moment capacity of the pipeline and bending strains the pipe will endure during the lay process; the deflection and/or curvature of the pipeline during the lay process; and the ability to hold the pipeline when laying (the vessel's top tension capacity) all have a significant role in determining the outcome of the pipe-lay process. The vessel's top tension is a key issue when laying a pipeline using a reeled pipe-lay method.
Other factors which are significant during the pipeline laying process include the lay vessel; the reeled capacity; the lay rate; the lay-down tension; the seabed condition and whether or not the pipe surface is irregular; lay water depth; the reeled pipe-lay vessel hold back tension; whether or not a vortex induced vibration may occur; the safety factors during the pipe lay process and the accessories it will be laid with (pig launchers or receivers, Pipe Line End Terminations (PLETs), Pipe Line End Manifolds (PLEMs), in line Y branches, or in line Tees.
Offshore pipelines are being installed in increasingly deeper water depths and the following table gives a sample overview.
Field nameOperatorLocationWater depthChinookPetroBrasGulf of Mexico2743 metresCascade(9000 ft)IndependenceAnadarcoGulf of Mexico2743NakikaShellGulf of Mexico2316CanyonTotal Fina ElfGulf of Mexico2195ExpressBlueStreamSaipem/KubanGazPromCaspian Sea2150BC 10ShellBrazil2000Thunder HorseBPGulf of Mexico1856GreaterBPAngola1400PlutonioAkpoTotalNigeria1375GumusutShellMalaysia1200Ormen LangeStatoilNorway1100
Offshore pipelines are at high risk of buckling during the initial laying operation. Most pipelines are laid containing an internal pressure of one atmosphere (1 Ata)
Tensioners on the pipe-lay vessel grip the pipeline being laid, by applying tension to the outside wall of the pipe. This gripping process can cause the circular shape of a thin walled pipe to alter and become more oval shaped. If the shape of the pipe is altered to an oval shape by as little as one wall thickness, the pipes resistance to buckling is reduced by as much as 40%. Consequently, much heavier walled pipe is used.
Other problems can also occur. Where a pipe meets the sea-bed, the pipe is subject to bending by its own weight, this occurrence is known as the SAG bend. The curvature at the SAG bend varies with pipeline lay tension. A buckle can start when tension forces at the SAG bend increase such that they cause the pipeline to bend where the natural curvature of the pipe is exceeded.
Once a buckle starts, it can propagate along a pipeline, flattening the pipeline until it reaches a buckle arrestor, a valve assembly, or shallower water, where the hydrostatic pressure exerting itself on the external pipeline wall is less than that of the buckling force.
The term ‘propagating buckle’ describes a phenomenon in which a buckle in an offshore pipeline changes its geometry from a transverse crease or dent to a longitudinal buckle and propagates along the pipe, causing the pipe to collapse along its length (or longitudinal axis). In a propagating buckle there are two distinct thresholds, the first threshold being the buckling pressure, which is the pressure at which a pipe under external pressure becomes unstable and buckles. The second threshold is the propagation pressure, which is defined as the lowest pressure at which an initiated buckle will propagate. The pressure required to maintain the propagating pressure, is less than the pressure required to initiate the buckle. Thus if a pipe buckles while subjected to an external pressure in excess of the initiation pressure, a propagating buckle will be formed that will advance along the pipe until a zone of less pressure is reached. When ‘Propagating Buckles’ occur, they follow a “fold up” U model, or dog-bone shape as they progress along a failing pipe.
Apart from during pipe lay installation activity, offshore pipelines which have a high outer diameter size to wall thickness relationship (OD to WT ratio) can buckle due to localised damage, whereby the damage is caused for example by dropped objects, anchor drags and/or seabed instability due to geological activity such as an earthquake.
Buckle arrestors have been used to safeguard a pipe against the catastrophic effects of a potential propagating buckle. There are a number of different types of buckle arrestors available, for example, one such buckle arrestor comprises full lengths or short sections of thicker pipe (thick walled rings), such arrestors are welded at periodic intervals into the pipeline being laid. Further examples of buckle arrestors include a thicker section of pipe wherein the section of pipe has an internal diameter that matches the external diameter of the pipe being protected. One or more of the thicker sections of pipe are clamped at periodic intervals onto the outside of the pipeline being protected. Alternatively, the buckle arrestor is slipped onto the pipeline and grouted during the lay process. Such buckle arrestors can only be used on lay barge type pipe-lay operations, as the pipe being laid from a reeled pipe-lay vessel does not have an end available to slip such arrestors over. The problem with a propagating buckle in a pipeline having a slip on, or bolted on external buckle arrestor is that the propagating buckle converts to a C shaped cross section, which passes through the arrestor. Slip on or bolted on buckle arrestors often cannot reach higher arresting efficiencies and their use in deep water is limited. Integrally welded heavy wall pipe joints make good buckle arrestors, but use of the heavier material introduces insurmountable reeling problems.
An example of a further buckle arrestor, is one made from fibre reinforced resins which are bonded to the pipeline exterior, after the pipeline is heated to 500F with an induction heater. Such buckle arrestors are made from spirally wound rods which are wrapped around pipelines. The ends of these rods are secured to the pipeline using common techniques known to the skilled person for example welding. The secured ends are then coated with a protective layer, usually cement before the pipe is laid.
Another type of buckle arrestor is a towed buckle arrestor. A towed buckle arrestor is pulled along inside the pipeline as the pipe is being welded and laid joint by joint. The buckle arrestor is secured using chains or wires to the lay-barge. The buckle arrestor cylinder is allowed to remain inside a section of pipe until that section reaches the sea-bed. Once the pipe touches the seabed the cylinder is pulled through to the open end of the next section of pipe. As each joint is laid, the cylinder is pulled along through the next joint. The problem with this method is that it does not lend itself to reeled pipe-lay, as there is no open end on the reeled pipe lay vessel.
Internal sleeves can be pre-installed inside the pipeline but these reduce the internal diameter of the pipeline and make it extremely difficult to pass an isolation tool such as a plug or a pigging tool.
A further example of a buckle arrestor is disclosed in the inventor's previous patent application WO 2005/028942. This device comprises a plurality of rubber wheels or tracks to engage with the interior surface of the pipeline to provide traction which enabled the device to move through the pipeline as desired by an operator. The problem with this device is that the reduced coefficient of friction in current internally coated pipelines, does not allow sufficient purchase to be attained by wheeled or tracked devices, which prevents the device from moving vertically up the pipeline as desired.